In electrophotography an image comprising a pattern of electrostatic potential (also referred to as an electrostatic latent image), is formed on a surface of an electrophotographic element comprising at least an insulative photoconductive layer and an electrically conductive substrate. The electrostatic latent image is usually formed by imagewise radiation-induced discharge of a uniform potential previously formed on the surface. Typically, the electrostatic latent image is then developed into a toner image by bringing an electrographic developer into contact with the latent image. If desired, the latent image can be transferred to another surface before development.
In latent image formation the imagewise discharge is brought about by the radiation-induced creation of electron/hole pairs, which are generated by a material (often referred to as a charge-generation material) in the electrophotographic element in response to exposure to the imagewise actinic radiation. Depending upon the polarity of the initially uniform electrostatic potential and the types of materials included in the electrophotographic element, either the holes or the electrons that have been generated, migrate toward the charged surface of the element in the exposed areas and thereby cause the imagewise discharge of the initial potential. What remains is a non-uniform potential constituting the electrostatic latent image.
Such elements may contain material which facilitates the migration of generated charge toward the oppositely charged surface in imagewise exposed areas in order to cause imagewise discharge. Such material is often referred to as a charge-transport material.
One type of well-known charge-transport material comprises a triarylamine. The term, "triarylamine," as used herein is intended to mean any chemical compound containing at least one nitrogen atom that is bonded by at least three single bonds directly to aromatic rings or ring systems. The aromatic rings or ring systems can be unsubstituted or can be further bonded to any number and any types of substituents. Such triarylamines are well known in the art of electrophotography to be very capable of accepting and transporting charges generated by a charge-generation material.
Among the various known types of electrophotographic elements are those generally referred to as multiactive elements (also sometimes called multilayer or multi-active-layer elements). Multiactive elements are so named, because they contain at least two active layers, at least one of which is capable of generating charge in response to exposure to actinic radiation and is referred to as a charge-generation layer (hereinafter sometimes alternatively referred to as a CGL), and at least one of which is capable of accepting and transporting charges generated by the charge-generation layer and is referred to as a charge-transport layer (hereinafter sometimes alternatively referred to as a CTL). Such elements typically comprise at least an electrically conductive layer, a CGL, and a CTL. Either the CGL or the CTL is in electrical contact with both the electrically conductive layer and the remaining CGL or CTL. The CGL comprises at least a charge-generation material; the CTL comprises at least a charge-transport material; and either or both layers may additionally comprise a film-forming polymeric binder.
Among the known multiactive electrophotographic elements, are those which are particularly designed to be reusable and to be sensitive to imagewise exposing radiation falling within the visible and/or infrared regions of the electromagnetic spectrum. Reusable elements are those that can be practically utilized through a plurality (preferably a large number) of cycles of uniform charging, imagewise exposing, optional development and/or transfer of electrostatic latent image or toner image, and erasure of remaining charge, without unacceptable changes in their performance. Visible and/or infrared radiation-sensitive elements are those that contain a charge-generation material which generates charge in response to exposure to visible and/or infrared radiation. Many such elements are well known in the art.
For example, some reusable multiactive electrophotographic elements which are designed to be sensitive to visible radiation are described in U.S. Pat. Nos. 4,578,334 and 4,719,163, and some reusable multiactive electrophotographic elements which are designed to be sensitive to infrared radiation are described in U.S. Pat. Nos. 4,666,802 and 4,701,396.
Many known reusable multiactive electrophotographic elements sensitive to visible or infrared radiation also employ triarylamine charge-transport materials in their CTL. In those elements the triarylamine is dispersed or dissolved in a film-forming polymeric binder that forms the CTL. Such elements are described, for example, in the four U.S. patents noted above. Those patents teach many polymers as having utility as film-forming binders for CTL's. Among the many polymers so described, are polycarbonates, such as poly[2,2-bis(4-hydroxyphenyl)propane carbonate] (commonly referred to as bisphenol A polycarbonate), and polyesters. Elements containing such components fairly adequately perform their intended functions, and, in the case of the elements described in the four U.S. patents noted above, have some very important advantages over other known elements. However, it has been recognized (e.g., in U.S. Pat. Nos. 4,840,860 and 4,840,861) that there are some significant drawbacks associated with such elements.
For example, if the CTL comprises a triarylamine in a bisphenol A polycarbonate film, a significant problem may arise. The problem can occur when the CTL has been adventitiously exposed to ultraviolet radiation (i.e., radiation of a wavelength less than about 400 nanometers, which, for example, forms a significant portion of the radiation emitted by typical fluorescent room lighting). This can occur, for example, when the electrophotographic element is incorporated in a copier apparatus and is exposed to typical room illumination during maintenance or repair of the copier's internal components. The problem, which has been referred to as a UV-fogging problem, is manifested as a buildup of residual potential within the electrophotographic element over time as the element is exercised through its normal cycles of electrophotographic operation after having been adventitiously exposed to ultraviolet radiation.
For example, in normal cycles of operation such an element might be initially uniformly charged to a potential of about -500 volts, and it might be intended that the element should then discharge, in areas of maximum exposure to normal imagewise actinic visible or infrared exposing radiation, to a potential of about -100 volts, in order to form the intended electrostatic latent image. However, if the electrophotographic element has been adventitiously exposed to ultraviolet radiation, there will be a buildup of residual potential that will not be erased by normal methods of erasing residual charge during normal electrophotographic operation. For example, after about 500 cycles of operation, the unerasable residual potential may be as much as -200 to -300 volts, and the element will no longer be capable of being discharged to the desired -100 volts. This results in a latent image being formed during normal operation, that constitutes an inaccurate record of the image intended to be represented. In effect, the element has become no longer reusable, after only 500 cycles of operation.
While the mechanism of this UV-fogging problem is not presently understood, U.S. Pat. Nos. 4,840,860 and 4,840,861 theorize that the problem may be caused by a chemical change in the triarylamine charge-transport material, induced by absorption of ultraviolet radiation. This is evidenced by an observed color change in the CTL after exposure to ultraviolet radiation. It would be desirable to be able to avoid or minimize this UV-fogging problem.
On the other hand, U.S. Pat. Nos. 4,840,860 and 4,840,861 have recognized that, if the electrophotographic element comprises a CTL, wherein the triarylamine is contained in a binder film of one of certain polyesters, the UV-fogging problem does not arise. Those patents theorize that this may be because the polyester absorbs more ultraviolet radiation than does a bisphenol A polycarbonate, and thus prevents some of the ultraviolet radiation from being absorbed by the triarylamine in amounts significant enough to cause the chemical change that leads to the UV-fogging problem, and/or the polyester or some complex of the polyester with the triarylamine may otherwise quench or prevent the UV-induced chemical change from occurring.
Unfortunately, such elements having a polyester as their CTL binder exhibit another drawback recognized in U.S. Pat. Nos. 4,840,860 and 4,840,861; namely, they have significantly lower sensitivity to actinic visible or infrared radiation (sometimes referred to as lower speed) than do elements that utilize bisphenol A polycarbonate as their CTL binder. For example, in some cases the exposure to actinic radiation necessary for discharging the initial uniform electrostatic potential from -500 to -100 volts (sometimes referred to as the 100-volt speed), is about 75 percent more when a polyester is the CTL binder, compared with when bisphenol A polycarbonate is the CTL binder. This is a very significant difference in terms of high speed copiers; i.e., the copier using polycarbonate as the CTL binder can make more than 5 exposures in the same time it takes the copier with the polyester CTL binder to make 3 exposures. It would, of course, be desirable to retain this speed advantage of the polycarbonate.
It thus became evident that there was a need for a reusable visible and/or infrared-sensitive electrophotographic element that avoids or minimizes the UV-fogging problem of elements utilizing a polycarbonate CTL binder, while at the same time avoiding or minimizing the speed loss inherent in elements utilizing certain polyester CTL binders.
The inventions described in U.S. Pat. Nos. 4,840,860 and 4,840,861 meet this need by providing electrophotographic elements wherein the CTL comprises binders that are mixtures of certain polycarbonates with certain polyesters. It was found that such mixtures synergistically provide most of the UV-fogging avoidance of certain polyesters while retaining most of the speed advantage of certain polycarbonates.
While those inventions provide great benefit, there are other drawbacks associated with them. Namely, the need to employ a mixture of two binder polymers in the same layer, rather than just a single binder polymer, requires that one be concerned with the compatibility of the polymers with each other and with charge-transport agents and any other materials desired to be included in a CTL. Any incompatabilities between such materials can result in phase separations during preparation or use of the electrophotographic element. Such phase separations can cause poorer or nonuniform electrical performance in the element and can cause undesirable scatter or absorption of actinic radiation during imagewise exposure, resulting in poorer image accuracy and resolution. The risk of this occurring is inherently greater when two polymers are employed instead of one.
Therefore, a need still existed for a binder polymer for a triarylamine-containing CTL, which would avoid the UV-fogging problem while enabling the electrophotographic element to exhibit better electrophotographic speed than is afforded by other polymers known to be useful for UV-fogging avoidance, and which would accomplish this without a need to be combined in a mixture with other polymers. The present invention satisfies this need.